Exercise training completed with low-carbohydrate reserves, via strategically-timed deprivation of carbohydrate intake, has been the topic of considerable scientific investigation in recent years. Research studies examining the consequences of ‘training low’ have been reviewed elsewhere (see the companion paper by E. Coleman on this site), and have generally concluded that training with low carbohydrate availability results in accentuated fat utilization, and increased markers of mitochondrial biogenesis (3, 7). Despite these presumably positive adaptations, ‘training low’ studies have not generally shown compelling evidence of improved performance in comparison to when training is completed with higher carbohydrate. Furthermore, perceived exertion during exercise is elevated when training is conducted in a glycogen-depleted state (13), and exercise tolerance and training capacity can be compromised when training with low carbohydrate availability (8, 17). Thus, although provocative recent evidence suggests that ‘training low’ may be beneficial under certain circumstances; it may not be desirable during sustained periods of heavy training. Therefore, this paper will examine the important role of dietary carbohydrate intake during heavy exercise training.
Prolonged and/or intense exercise results in a variety of physiological outcomes, including muscle glycogen depletion, dehydration, altered muscle protein turnover, physical and mental fatigue, etc. These responses are natural consequences of the training stimuli necessary to elicit adaptations to training. High-level athletes commonly perform sustained periods of intensified training (i.e. training camps, or intensified blocks of training), in which training demands are substantially increased in comparison to normal training levels. These periods of heavy training often result in symptoms of ‘overreaching’, characterized by decrements in exercise performance, worsened mood and fatigue, and altered concentrations of stress hormones such as cortisol (1, 6, 10).
Although intensified training can result in the short-term impairments described above, the strategy is commonly utilized by competitive athletes, with the goal of promoting positive training adaptations when the heavy training is followed by appropriate periods of recovery. For example, Coutts and colleagues (5) subjected triathletes to a four-week period of intensified training, followed by a two-week taper. Running performance (during a 3 km time trial) was impaired by 4% following the period of intensified training, but was improved 7% following the subsequent taper period. Thus, the program was effective at promoting gains in performance over the 6-week period; a phenomenon best described as ‘functional overreaching’ (12). In practice, it can be challenging to effectively manage the influences of training stimulus and recovery. If training loads are not balanced with appropriate recovery, ‘non-functional overreaching’ may result; leading to longer-term decrements in exercise performance (and other symptoms) which continue after a planned period of reduced training. This requires coaches/athletes to implement longer (unintended) periods of recovery to restore performance (12). As discussed below, carbohydrate intake plays an important role in recovery and exercise tolerance; improving the likelihood of positive adaptations to heavy training.
The ergogenic effects of carbohydrate consumed during endurance-related exercise have been extensively studied since the 1980's. Although a detailed review of this topic is beyond the scope of this paper [see recent review from Saunders and Luden (14)], carbohydrate ingestion during exercise maintains euglycemia, spares hepatic (and possibly skeletal muscle) stores of glycogen, sustains high rates of carbohydrate oxidation throughout exercise, and may attenuate central nervous system fatigue. As a result, carbohydrate ingestion has been consistently shown to augment performance during prolonged performance trials (> 2 hours), and may also improve performance during shorter/intense endurance exercise (i.e. 1 hour at ~85-90% VO2max).
In addition to its acute ergogenic effects, carbohydrate intake has been shown to promote recovery from endurance exercise, thereby improving exercise tolerance and potential adaptations to heavy training. For example, repeated days of heavy exercise may result in inadequate resynthesis of muscle glycogen stores (2, 4, 15). However, the consumption of high carbohydrate diets (8-10 g/kg body weight (kgBW)) during heavy training have been reported to increase glycogen levels in comparison to lower carbohydrate diets (4-5 g/kgBW) (15, 16). Higher muscle glycogen content allows increased rates of carbohydrate utilization during subsequent exercise (1, 2), and has been shown to improve performance in some (2, 16), but not all studies (15). This phenomenon may also be responsible for the impaired exercise capacity reported in some ‘training low’ studies. Specifically, self-selected workloads during high-intensity interval training have been significantly lower when exercise was conducted with low-carbohydrate availability (8, 17).
Training with higher carbohydrate intake reportedly enhances tolerance to intensified training. For example, Achten and colleagues (1) examined the effects of 1 week of intensified distance running when athletes consumed varying amounts of dietary carbohydrate (5.4 g/kgBW vs. 8.5 g/kgBW). Higher carbohydrate intake mitigated declines in mood states and fatigue, prevented declines in carbohydrate/glycogen oxidation during subsequent exercise, and resulted in better maintenance of running performance over the intensified training period in comparison to lower carbohydrate intake.
Carbohydrate intake during (11) and immediately following exercise (9) may be particularly important for optimal carbohydrate oxidation and glycogen replenishment, respectively. For example, Ivy and colleagues (47) observed that glycogen replenishment rates were higher when carbohydrate was fed immediately post-exercise, versus when feedings were delayed. Thus, it is likely that the consumption of higher carbohydrate doses during these specific times may be especially useful during heavy training. Halson and associates (6) investigated the effects of altered carbohydrate intake during/following exercise over the course of 8-days of intensified training. Specifically, cyclists received low- or high-carbohydrate feedings during exercise (2% or 6% carbohydrate solutions, respectively) and immediately following each exercise session (2% or 20% carbohydrate solutions), with no additional manipulations to training diets. Symptoms of overreaching were observed after intensified training with both diets; including reductions in time-to-fatigue (at ~75% VO2max), increased perceived effort during exercise, worsened mood states, and lowered cortisol and epinephrine responses to exercise. However, each of the aforementioned effects was attenuated in the high-carbohydrate condition. Importantly, the intensified training phase was immediately followed by 14 days of reduced-volume training, with continued nutritional intervention. Many of the differences between nutritional conditions (i.e. lower perceived exertion, and improved mood states with higher carbohydrate intake) persisted throughout this recovery phase. Following recovery, subjects improved their performance (time-to-fatigue) by >10% with high-carbohydrate feedings (versus baseline levels; i.e. functional overreaching), but remained ~15% below baseline levels with low-carbohydrate (non-functional overreaching). This highlights the importance of carbohydrate intake on exercise tolerance, and adaptations to intensified training — particularly when consumed during and immediately following exercise.
In summary, recent evidence suggests that training with low carbohydrate availability may promote positive physiological adaptations under certain circumstances. However, higher carbohydrate intakes are recommended for endurance athletes during periods of heavy training, and when peaking for important competitions. Adequate carbohydrate intake is associated with better exercise tolerance to heavy training, and appears to be critical for sustaining high rates of carbohydrate oxidation for high-level endurance performance.